For many pressure swing adsorption (PSA) separations, vacuum regeneration of the one or more adsorbent beds will significantly improve PSA process performance. The major drawback of vacuum regeneration, however, has been the high energy costs associated with the generation of a vacuum. The present invention eliminates or reduces this problem by using the energy available in the usually discarded vent gas to create a source of vacuum. In particular, the relatively high pressure of the vent gas is used to transfer a portion of dense liquid from a lower tank to a higher tank. When this same portion of dense liquid is returned to the lower tank, a vacuum is created which can be used to regenerate an adsorbent bed.
In the following description of the invention, the term "pressure swing adsorption" or its acronym "PSA" is used in reference to a type of process and apparatus that is now well known and widely used with respect to separating the components of a gaseous mixture. A PSA system basically comprises passing a feed gas mixture through one or more adsorption beds containing a sieve material which has a greater selectivity for a more strongly adsorbed component than a more weakly adsorbed component of the gas mixture. In the operation of a typical 2-bed PSA system, the connecting conduits, valves, timers, and the like are coordinated and arranged so that when adsorption is occurring in a first bed, regeneration is occurring in a second bed. In the usual cycle, sequential steps with respect to each bed include bed pressurization, product release and venting. Basic PSA systems are described in U.S. Pat. Nos. 2,944,627, 3,801,513, and 3,960,522.
Various modifications and improvements to the basic PSA process and apparatus have been described in various patents, for example, in U.S. Pat. No. 4,415,340, issued on Nov. 15, 1983 and U.S. Pat. No. 4,340,398 issued on July 20, 1982. The present invention is not limited to the use of any particular PSA process or apparatus design.
Venting an adsorbent bed is the simplest method of regenerating a PSA bed for further adsorption. Other combined modes of regeneration which are conventionally used to improve the regeneration step include (i) regeneration at atmospheric pressure coupled with product purge, and (ii) vacuum regeneration. An advantage of product purge is that relatively low energy is required for regeneration. A disadvantage of product purge is that a relatively low product yield is obtained as a result of the loss of the product purge gas itself.
The yield obtained using vacuum regeneration is generally superior to the yield using product purge. Vacuum regeneration, however, increases the capital investment for the process slightly and the energy requirement appreciably. Consequently, it has been necessary to weigh the increase in product yield that results with vacuum regeneration against the incremental cost that is primarily associated with energy charges. It would therefore be desirable to reduce the energy requirements of vacuum regeneration in order to more fully benefit from the superior yields associated with vacuum regeneration.
It is therefore an object of the present invention to increase the yield of a PSA system by using vacuum regeneration, while reducing the high energy costs typically associated with vacuum regeneration relative to other modes of regeneration.
It is a further object of the present invention to harness the energy of the usually discarded vent gas in a PSA process to create a potential source of energy for creating a vacuum.
It is yet a further object of the invention to use the vent gas pressure in a PSA process to transfer a dense liquid from a lower tank to a higher tank and, furthermore, to create a source of vacuum by returning the transferred liquid back to the lower tank which is subsequently utilized while the higher tank is in communication with the adsorbent bed.